Technologies such as microelectronics, micromechanics and biotechnology have created a high demand for structuring and probing specimens within the nanometer scale. On such a small scale, probing or structuring is often performed with charged particle beams which are generated and focused in charged particle beam devices. Examples of charged particle beam devices are ion microscopes as well as ion beam pattern generators. Charged particle beams, in particular ion beams, offer superior spatial resolution compared to photon beams, due to their short wave lengths at comparable particle energy.
Typically, a charged particle beam device for generating an ion beam comprises an ion point source and an optical column. Examples for ion point sources are liquid metal ion sources and gas field ion sources. In liquid metal ion sources, ion formation results from field desorption from a liquid protrusion stabilized by the electric field. Hereto in contrast, ion formation in a gas field ion source is based on ion formation at the very tip of an emitter unit in a gas ambient. The emitter tip is biased at e.g. 10 kV positive with respect to a downstream extraction aperture that produces an electric field strong enough to ionize the gas atoms in the vicinity of the emitter unit.
Gas field ion sources provide advantages over liquid metal ion sources including pumping out the gas after its neutralization, thereby preventing deposition, and decreasing the size of the ion beam by a very monochromatic ion beam. Further, in contrast with liquid metal ion sources, beams of light ions such as Helium ions or the like can be generated.
However, gas field ion sources also provide difficulties. One of the main difficulties is to get enough gas particles to the tip without increasing the pressure in the optical column because in this case the gas would downgrade the vacuum level in the high-vacuum optical column. This, in turn, would result in a higher rate of collisions of the beam ions with the residual gas, thus leading to blur in the irradiating application. For example, it is known in the state of the art to operate the emitter tip within a recipient with a high partial pressure of the utilized gas, and to apply a differential pumping system to isolate this recipient from the rest of the optical column. It is further known in the state of the art to use a nozzle that directs the gas flow to the emitter tip. This reduces the total gas load as only the area around the emitter tip is exposed to the required high gas pressure. For instance, in a geometry, where the nozzle with a 100 μm opening is at a distance of 10 mm from the tip, and where the minimum pressure required near the tip is about 10−4 mbar, the required gas flow dynamics results in an aperture angle of the gas beam of about 30° half width (cosine distribution) so that still a rather large total gas flow results. This will be explained in more detail with regard to FIGS. 1a and 1b below. In general, ion generating would capitalize from pressures of up to 10−2 mbar and more. However, if state of the art ion sources are operated at pressures of 10−3 mbar, 10−2 or higher pressures, the ion beam quality is substantially reduced due to the large number of gas particles contaminating the optical column. Examples for gas field ion sources as described in this paragraph are given in H. Horishima et al: “A focused He+ ion beam with a high angular current density,” Jpn. J. Appl. Phys. Vol. 31 (1992), pp. 4492-4495; E. Salancon et al: “A new approach to gas field ion sources,” Ultramicroscopy 95 (2003), pp. 183-188, and W. Thompson et al: “The gas field ion source for finely focused ion beam systems,” Mat. Res. Soc. Symp. Proc. Vol. 396 (1996), pp. 687-693.
High gas load in the source leads to glow discharge. A further improvement known in the state of the art is to use one nozzle for the gas supply and a further nozzle to pump this gas. By applying this technique, the total gas load can be further minimized. An example of such an ion beam generating apparatus is given in U.S. Pat. No. 4,638,209.
However, the techniques known in the state of the art are still problematic in that too many residual gas particles get into the optical column and cause collisions with the beam ions. As this leads to blur, the quality of the irradiating application is jeopardized.
The documents EP 1 439 564 A1, J. Guevremont et al.: “Design and characterization of collimated effusive gas beam sources: Effect of source dimensions and backing pressure on total flow and beam profile,” Rev. of Sc. Instrum. Vol. 71 (2000), and C. Lucas: “The production of intense atomic beams,” Vacuum 23 (1972), deal with gas flows. However, none of them deals with the specific difficulties of liquid gas ion sources.
Accordingly, it is an object of the present invention to overcome at least part of the problems in the state of the art. It is particularly an object of the present invention to provide a charged particle beam apparatus having a gas field ion source and a method of operating thereof that minimizes the interactions of the ion beam with gas molecules.